A. Field of the Invention
This invention relates generally to meter circuitry for electronic systems, and more particularly to a meter driving circuit for metering both a DC and AC operation of a radio circuit stage.
B. Description of the Prior Art
In radio circuits it is common in the prior art to build into selected stages a metering circuit for indication of AC signal levels within the selected stage. The output of this circuit is commonly coupled to a test point or a metering socket. This is done to facilitate testing and maintenance procedures which have to be performed on radio equipment.
Typically, in the prior art, this function is accomplished by coupling the AC signal from some point in the selected stage to the cathode of a diode detector. This point is also coupled to a meter test point while the anode of the diode detector is coupled to ground thereby forming a rectifying circuit which permits metering of RF or audio signal levels.
This approach has a distinct disadvantage in that the AC signal to be detected must have a sufficient voltage swing to overcome the threshold potential (barrier potential) of the diode detector before a signal can be detected. This limits the sensitivity of the metering circuit. To minimize this problem, a germanium diode is commonly used since it has a smaller barrier potential than a silicon diode. In addition, it is often desirable to be able to test the DC bias conditions of the stage as well. However, in the prior art, this would require a separate test point and the performance of a separate test.
This invention is made based on consideration of the above problems and is intended to solve both the sensitivity problem and to provide for metering of both AC and DC conditions with one test point.